Auth0
Normally you just collect username or email and password from the user and store it in a database and verify on the backend. But Auth0 is a platform that basically does all of that for you? There also multi-factor authentication and social logins like apple and google that use the openID-Connect standard.
Its a security standard that allows you to access data in another application. Instead of giving them a username password, you give them an access key. 1. authorization 2. delegated authorization
OAuth Flow
visible steps to grand consent from the user.
openID-Connect
Auth0 is only authentication, openID is basically a user profile, basically Single-Sign-on(SSO). So it gives both an access token and an Identity token.